Gill Morrell is one of the stable of Stabenfeldt authors. She is British, and was born near London, and grew up longing to ride. She has written three titles that I have been able to find: all as far as I can see published by Stabenfeldt. The Pony Vacation was previously published as The Pony Holiday, but I can find no record of it anywhere. Her three books form a series about Jess, and her riding holidays.
Finding the books
No Stabenfeldt titles were published in the UK, but some copies have been imported and are usually available fairly cheaply here. Alternatively, you can try American sites, or Ebay.com, where bundles of titles tend to come up fairly often.